1. Field of the Invention
The present invention relates to an optical encoder with an improved construction of the optical detection unit. The present invention may be applied to optical encoders of either the rotary type or linear type.
2. Description of the Related Art
Optical encoders are widely used to detect the position and speed of objects, such as motors, performing rotational movement or translational movement. FIG. 9A-FIG. 9C show an example of a mobile code plate and a photodetector unit constituting the optical detection unit of a rotary optical encoder.
Mobile code plate 1 is produced by evaporating chromium or the like on to a glass plate, then forming transparent sections 11, 12, 13a, 13b, 14a, 14b of concentric arcuate shape by etching. The regions of evaporated film of chromium or the like that are left behind (hatched regions in FIG. 9A) constitute optical screening parts.
A light source unit 2 and photodetector unit 3 are provided on either side of mobile code plate 1. Light source unit 2 and photodetector unit 3 are constituted respectively by the required number of light-emitting elements 21, 22 and photodetector elements 31 to 34.
FIG. 9B shows a cross-sectional view when mobile code plate 1 is in the condition of FIG. 9A; FIG. 9C shows a cross-sectional view when mobile code plate 1 has rotated through 90.degree. from the condition of FIG. 9A in the direction of arrow A.
When transparent parts 11, 12, 13a, 13b, 14a, 14b arrive at a position directly in front of photodetector elements 31 to 34, the light (incoming light) that is directed on to mobile code plate 1 from light emitting elements 21, 22 of light source unit 2 is incident on the corresponding photodetector elements 31 to 34, causing a signal expressing detection of the light to be output. In the condition of FIG. 9B, respective transparent parts 14a, 12 have arrived in positions facing photodetector elements 31, 33, allowing light from light emitting elements 21, 22 to be propagated straight through transparent parts 14a, 12 until it is respectively incident on photodetector elements 31, 33. No transparent part has arrived in the positions facing photodetector elements 32, 34, so light from light emitting elements 21, 22 is screened, and there is no optical input to photodetector elements 32, 34.
In contrast to this, in the condition of FIG. 9C, transparent parts 13b, 11 have respectively arrived in the positions facing photodetector elements 32, 34, so light from light emitting elements 21, 22 is propagated straight through transparent parts 13b, 11 and is respectively incident on photodetector elements 32, 34. No transparent part has arrived in the positions facing photodetector elements 31, 33, so light from light emitting element 21 is screened, and there is no optical input to photodetector elements 31, 33.
When mobile code plate 1 that is mounted on the subject of detection (for example the rotary shaft of a motor) rotates in direction A or in direction B about rotary axis X--X, a photodetection signal is cyclically output from photodetector elements 31.about.34. The output signals from photodetector elements 31 to 34 are processed by a well-known circuit, not shown, and the rotational position and rotational speed etc. of the subject of detection are thereby detected.
With an optical encoder according to the prior art as described above, it is necessary to form optical screening film such as chromium and remove the optical screening film by performing etching in a prescribed code pattern. Furthermore, since, for the etching, chemicals whose management and processing are troublesome are employed, production costs are high and the method is unsuitable for mass production. A further problem is that, since optical encoding is performed by simple transmission/screening of the light, this inevitably resulted in light being uselessly discarded during screening.